This invention relates generally to various types of seal arrangements. Such seals generally are utilized in spindles for sealing the gap between a fixed outer housing or enclosure and a rotatably mounted shaft extending through and from the housing. Most spindle arrangements can operate over a wide range of applications without breaking down and for long periods of time only if protected by effective sealing arrangements during their service life whereby dirt, liquids and other contaminants are prevented from penetrating into the shaft housing. It is also important in many applications to prevent lubricants from escaping from within the shaft housing.
A multitude of different designs of shaft seals are known and described in standard engineering and production literature. Such shaft seals are generally classified as either contacting or non-contacting.
Contacting seals refer to seals wherein sealing surfaces rub against the rotating machine component with a predetermined pressure and are usually used in sealing against the penetration of foreign substances into the spindle housing. However, due to the contact pressure which exists and the frictional heat generated thereby, the use of these bearings at high speeds is somewhat limited. Thus, contact bearings tend to lose their effectiveness either through wear due to friction or through the heat generated by the sliding contact. The increases in temperature caused by the frictional sealing contact additionally affects adjacent machine components such as the bearings in which the shaft is mounted or even the entire spindle system.
The service life of bearings and lubricants is significantly reduced when the bearings operate at high temperatures.
The maximum possible peripheral speed of contacting seals of various designs is limited depending upon the structural shape and materials of these seals as well as the environmental conditions under which the seals operate. The lubrication and construction of the sealing lip on which the rotating sealing surface is provided also affects the range of speed over which the bearing is operable.
Generally, the upper limit of peripheral speed for a contacting seal is about 30 m/s. At such high speeds, the temperatures to which the bearing surfaces are subjected due to the friction created by the contacting seal results in heating of the bearing and spindle system and in consequence dimensional changes which may substantially impair the accuracy and operation of the machine.
Non-contacting seals have therefore been used in high-speed spindles. Such non-contacting seals have very narrow, smooth, radial or axial gaps, or so-called labyrinth gaps. However, such non-contacting seals cannot function over wide ranges of application.
Usually, in addition to the non-contact seal construction, a shield disc is fixed to the shaft in front of the non-contacting seal. A shield disc rotates with the shaft and thus prevents the entry of dirt and liquid into the housing by centrifugal force. The effect of this on a non-contacting seal which is generally determined by the length of the sealing gap or the number of labyrinths is often quite limited due to the small area available in which the spindle is to be mounted. In order to obtain a rigid spindle the distance between the outer point at which the forces are applied to the end of the spindle shaft and the bearing point at which the shaft is supported in the beaaring must be as short as possible.
In certain applications significant amounts of possible contaminants are present during rotation of a rotating spindle shaft. For example, in grinding operations liquid coolant and substantial material waste are present during the grinding process. In such applications it is not uncommon to use a gaseous medium, for example compressed air alone or with an oil mist entrained therein, to intensity the sealing effect. If, however, the supply of compressed air is terminated when the equipment operation stops, coolant or other contaminants from the working area may enter into the spindle housing and through the sealing gap or labyrinth seal.
The penetration of coolant into the spindle housing is also possible when the spindle shaft rotataes at relatively low speeds since the centrifugal force acting on the shield disc is not very effective. The danger is especially prevalent if there is a failure in the supply of the compressed sealing air.
Particularly in connection with modern high-speed cutting techniques for metallic materials as well as wood, stone and semiconductor substrate processing for electronics applications, very large quantities of coolant must be present in the tool contact zone to dissipate the heat generated during cutting and to facilitate the discharge of chips. The large quantity of coolant, which frequently must be supplied under high pressure, requires an improved sealing of the workpiece spindle against the penetration of the coolant and other contaminants.